[PATCH v3 4/8] dma-mapping: Introduce DMA require coherency attribute

Leon Romanovsky
2026-03-16T19:07:24Z

The mapping buffers which carry this attribute require DMA coherent system.
This means that they can't take SWIOTLB path, can perform CPU cache overlap
and doesn't perform cache flushing.

Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
---
Documentation/core-api/dma-attributes.rst | 16 ++++++++++++++++
include/linux/dma-mapping.h | 7 +++++++
include/trace/events/dma.h | 3 ++-
kernel/dma/debug.c | 3 ++-
kernel/dma/mapping.c | 6 ++++++
5 files changed, 33 insertions(+), 2 deletions(-)

diff --git a/Documentation/core-api/dma-attributes.rst b/Documentation/core-api/dma-attributes.rst
index 48cfe86cc06d7..441bdc9d08318 100644
--- a/Documentation/core-api/dma-attributes.rst
+++ b/Documentation/core-api/dma-attributes.rst
@@ -163,3 +163,19 @@ data corruption.

All mappings that share a cache line must set this attribute to suppress DMA
debug warnings about overlapping mappings.
+
+DMA_ATTR_REQUIRE_COHERENT
+-------------------------
+
+DMA mapping requests with the DMA_ATTR_REQUIRE_COHERENT fail on any
+system where SWIOTLB or cache management is required. This should only
+be used to support uAPI designs that require continuous HW DMA
+coherence with userspace processes, for example RDMA and DRM. At a
+minimum the memory being mapped must be userspace memory from
+pin_user_pages() or similar.
+
+Drivers should consider using dma_mmap_pages() instead of this
+interface when building their uAPIs, when possible.
+
+It must never be used in an in-kernel driver that only works with
+kernal memory.
diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
index da44394b3a1a7..482b919f040f7 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -86,6 +86,13 @@
*/
#define DMA_ATTR_DEBUGGING_IGNORE_CACHELINES (1UL << 11)

+/*
+ * DMA_ATTR_REQUIRE_COHERENT: Indicates that DMA coherency is required.
+ * All mappings that carry this attribute can't work with SWIOTLB and cache
+ * flushing.
+ */
+#define DMA_ATTR_REQUIRE_COHERENT (1UL << 12)
+
/*
* A dma_addr_t can hold any valid DMA or bus address for the platform. It can
* be given to a device to use as a DMA source or target. It is specific to a
diff --git a/include/trace/events/dma.h b/include/trace/events/dma.h
index 8c64bc0721fe4..63597b0044247 100644
--- a/include/trace/events/dma.h
+++ b/include/trace/events/dma.h
@@ -33,7 +33,8 @@ TRACE_DEFINE_ENUM(DMA_NONE);
{ DMA_ATTR_NO_WARN, "NO_WARN" }, \
{ DMA_ATTR_PRIVILEGED, "PRIVILEGED" }, \
{ DMA_ATTR_MMIO, "MMIO" }, \
- { DMA_ATTR_DEBUGGING_IGNORE_CACHELINES, "CACHELINES_OVERLAP" })
+ { DMA_ATTR_DEBUGGING_IGNORE_CACHELINES, "CACHELINES_OVERLAP" }, \
+ { DMA_ATTR_REQUIRE_COHERENT, "REQUIRE_COHERENT" })

DECLARE_EVENT_CLASS(dma_map,
TP_PROTO(struct device *dev, phys_addr_t phys_addr, dma_addr_t dma_addr,
diff --git a/kernel/dma/debug.c b/kernel/dma/debug.c
index 83e1cfe05f08d..0677918f06a80 100644
--- a/kernel/dma/debug.c
+++ b/kernel/dma/debug.c
@@ -601,7 +601,8 @@ static void add_dma_entry(struct dma_debug_entry *entry, unsigned long attrs)
unsigned long flags;
int rc;

- entry->is_cache_clean = attrs & DMA_ATTR_DEBUGGING_IGNORE_CACHELINES;
+ entry->is_cache_clean = attrs & (DMA_ATTR_DEBUGGING_IGNORE_CACHELINES |
+ DMA_ATTR_REQUIRE_COHERENT);

bucket = get_hash_bucket(entry, &flags);
hash_bucket_add(bucket, entry);
diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
index 3928a509c44c2..6d3dd0bd3a886 100644
--- a/kernel/dma/mapping.c
+++ b/kernel/dma/mapping.c
@@ -164,6 +164,9 @@ dma_addr_t dma_map_phys(struct device *dev, phys_addr_t phys, size_t size,
if (WARN_ON_ONCE(!dev->dma_mask))
return DMA_MAPPING_ERROR;

+ if (!dev_is_dma_coherent(dev) && (attrs & DMA_ATTR_REQUIRE_COHERENT))
+ return DMA_MAPPING_ERROR;
+
if (dma_map_direct(dev, ops) ||
(!is_mmio && arch_dma_map_phys_direct(dev, phys + size)))
addr = dma_direct_map_phys(dev, phys, size, dir, attrs);
@@ -235,6 +238,9 @@ static int __dma_map_sg_attrs(struct device *dev, struct scatterlist *sg,

BUG_ON(!valid_dma_direction(dir));

+ if (!dev_is_dma_coherent(dev) && (attrs & DMA_ATTR_REQUIRE_COHERENT))
+ return -EOPNOTSUPP;
+
if (WARN_ON_ONCE(!dev->dma_mask))
return 0;

--
2.53.0